I had a flat tire a while ago, so I mounted the space-saver spare. I'll admit I left it on for way longer than the owner's manual suggests. Eventually, I replaced it with a full-size tire, but I've always wondered why manufacturers say you shouldn't use the spare for longer than necessary.

Since the spare tire is used so infrequently, carmakers have switched to narrow, compact spares to save space and weight. Of course, a spare tire is a lifesaver when regular tires go pop, but leaving the temporary tire on for longer than the manufacturer recommends invites a host of problems. First, a temporary spare isn't as durable as a normal tire. The real strength of a tire comes from the plieslayers of steel and polyester underneath the rubberand spares don't have as many plies as regular tires. A typical space-saver spare has only one layer of polyester in the sidewall and two belts of steel with a layer of polyester in the treadabout half as many plies as a normal tire. This greatly limits puncture resistance and cornering ability. In addition, as the name implies, space-saver tires are intended to take up less room in car and crossover trunks so that those trunks can be deeper. For that reason, these tires are narrower and have a smaller contact patch. This reduces the amount of traction for the tire, increasing stopping distances and making handling potentially unpredictable in emergency maneuvers. It also means ABS and traction control aren't as effective at keeping you out of danger. And you're not going to have the same ground clearance. If you're towing a trailer, you'll have to leave it behindspares have much lower load ratings than regular tires. Long-term use of the spare can cause a serious mechanical issue, too: The smaller-diameter tire can put a lot of stress on your differential.

The differential has a tricky job. It transmits engine power to the wheels from the transmission, but it also lets the left and right wheels turn at different speeds. This is essential for cornering. In a turn, the path of the inside wheel is shorter than that of the outside wheel, which means they travel at different speeds. When your car is driving in a straight line, the differential isn't in use and there's little wear and tear on its gears and bearings. But because the spare is smaller than the opposing wheel on the same axle, it must turn faster to keep up with the speed of the car, making the differential work to account for the variation. It's as if the car is constantly in a turn. Leave the spare on long enough and the grease lubricating the differential will begin to break down, accelerating wear between the gears and the clutch plates if it's a limited-slip differential. For all these reasons, manufacturers suggest keeping speeds below 50 mph and using the spare tire only for limited distances if possible. If a compact spare is ever damaged, either the tire itself or the wheel, the entire spare should be replaced rather than repaired. And don't forget to check the pressure in your spare every time you check the pressure in your other tiresit's important to make sure your safety net is, in fact, safe.

This content is created and maintained by a third party, and imported onto this page to help users provide their email addresses. You may be able to find more information about this and similar content at piano.io

This commenting section is created and maintained by a third party, and imported onto this page. You may be able to find more information on their web site.

A Part of Hearst Digital Media
Popular Mechanics participates in various affiliate marketing programs, which means we may get paid commissions on editorially chosen products purchased through our links to retailer sites.